Selective ionization detector

ABSTRACT

Selective ionization detector for halogen, phosphorus or nitrogen compounds of the type including a diode through which a sample gas being analyzed is fed by means of a carrier gas and an electrode including an alkali source in the form of a heated alkali-containing glass so that the electrode exhibits an increased ion emission upon the occurrence of such specific substances. The major improvement comprises maintaining the alkali glass in a heated softened state during operation of the detector. This may be accomplished by an electric resistance heater, the electric energy to which is precisely adjusted; or by the flame of a burner to which the sample gas and a combustible gas are supplied. The alkali-supplying glass is maintained negative with respect to the collecting electrode, while the (burner nozzle) emitting electrode may be maintained at the same potential as the glass body or also at a positive voltage relative thereto.

Dec.3, 1974 SELECTIVE IIONIZATION DETECTOR [75] Inventors: Bruno Kolb,Owingen; Joachim Primary Examiner-Robert Bischofi, Uberlingen, both ofAttorney, Agent, or Firm-Daniel R. Levmson Germany 57 ABSTRACT Selectiveionization detector for halogen, phosphorus or nitrogen compounds of thetype including a diode [73] Assignee: Bodenseewerk Perkin-Elmer & Co.

Gmbl-l., Uberlingen/Bodensee,

Germany through which a sample gas being analyzed is fed by [22] Filed:May 4, 1973 means of a carrier gas and an electrode including an alkalisource in the form of a heated alkali-containing [21] Appl' 357496 glassso that the electrode exhibits an increased ion emission upon theoccurrence'of such specific sub- [30] Foreign Application Priority Datstances. The major improvement comprises maintain- May 6, 1972 Germany2222396 ing the aikaii giass a heated softened State during operation ofthe detector. This may be accomplished 52 vs. C] 23/254 EF by hie-Chicresistance heater the electric energy to 51 int. Cl. 001 31/12 which ispificiseiy adjusted; or by the flame of a [58] Field of Search 23/254EF, 232 0 burner to which the Sample 8 and Combustible gas are supplied.The alkali-supplying glass is maintained 56] References Cited negativewith respect to the collecting electrode, while the (burner nozzle)emitting electrode may be main- UNITED T T PATENTS tained at the samepotential as the glass body or also at a positive voltage relativethereto. armen t i 3,589,869 6/1971 Scolnick 23/254 EF 8 Claims, 6Drawing Flgures PATENTELBEC 3 M mm? sum 2 BF 3 Fig. 6

PATEN IL EZC 31874 sum 3 or 3 Fig.4

SELECTIVE IONIZATION DETECTOR This invention relates to a selectiveionization detector for halogen, phosphorus or nitrogen compounds,comprising a diode through which a sample gas under analysis can be fedby means of a carrier or transfer gas, and which comprises an electrodeincluding an alkali source in the form of a heated alkali-containingglass so that the electrode exhibits an increased ion emission upon theoccurrence of such specific substances.

A prior art selective ionization detector of this type which isparticularly intended for leak detecting devices or the like includestwo coaxially helically wound wires each of which are heatable by meansof a filament transformer winding. Between the two wires, whichconstitute the electrodes of a diode, a d.c. voltage source is connectedin series with a measuring device. The innermost wire is wound onto acylinder of alkali metal glass, for instance, potassium glass. By meansof the alkali metal of the glass cylinder a specific sensitizing of theelectrode for halogen is effected so that upon passage ofhalogen-containing gases or vapors between the electrodes a definiteincrease in the current flowing across the diodes is observed (Germanpatent specification No. 907,223).

It is also prior art to use such detectors in gas chromatography (Germanpatent specification No. 1,149,924). These detectors have recentlyobtained particular significance in the analysis of pesticide residuesin vegetable or animal substances, or of medicinal or drug residues inthe blood, sweat or urine.

Moreover, detectors of the above type are prior art in which anelectrode and/or a separate supply of alkali metals are heated by meansof a flame. These detectors are formed in the same manner as a flameionization detector, which detector is conventional in gaschromatography. A burner nozzle is provided to which a combustible gas(for example, hydrogen) and sample gas mixture is supplied, with thesample gas in gas chromatographic detector applications again being amixture of carrier gas plus eluted sample components. Above the flame oraround the flame there is arranged a collecting electrode. Around theflame an electrode in the form of a spiral is arranged which is coatedwith a layer of molten alkali salt, preferably sodium sulphate (see U.S.Pat. No. 3,372,994) for sensitizing.

With such flame ionization detectors an increased selective sensitivityfor halogen and phosphorus is obtained. However, they are also sensitiveto many other substances at the normal sensitivity of-a flame ionizationdetector. Sometimes, this is disturbing. Therefore, it is also known toburn the sample substances prior to measurement in the selectivedetector either at a separate flame ionization detector (Germanpublished patent application No. 1,598,] 18) or by flameless oxidation(German published patent application No. 1,598,132).

All these known detectors share the problem that the alkali metalnecessary for proper functioning should be made available uniformly,i.e., constant in time, within the detector.

It is prior art to apply alkali salts to a metal grid in a flameionization detector (Journal of Gas Chromatography, volume 3 (1965),pages 336-339). This method has not been completely successful as theamount of alkali salts decrease markedly after hours of use, and thedetectorconstantly decreases in sensitivity. A similar phenomenon isobserved in the arrangement of the type mentioned hereinbefore (Germanpatent specification No. 907,223) in which a heated coil is wound as anelectrode around a cylinder of an alkali glass.

Moreover, alkali salts have been provided in supply vessels of aperturedor porous materials so that by diffusion over a longer period of timealkali metal compounds are passed to the surface of these supply vessels(German published patent application No. 1,598,] 18). Although thismethod ensures an alkali supply for days or weeks, the decrease in thedetector sensitivity is still disturbingly high. Moreover, these supplyvessels are expensive to manufacture.

It is also prior art to attach a supply of a salt of analkali metal tothe burner nozzle of a flame ionization detector (German publishedpatent application No. 1,900,981 In another prior art selective flameionization detector, a solid piece of alkali salt is held in the flameby means of a mount. The mount permits change in the spatial position ofthe alkali salt piece relative to the flame nozzle (German publishedpatent application No. 1,935,624). Nitrogen-containing substances canalso be detected selectively by these latter detectors. However, puresalts are brittle and have only poor adhesion to other materials, sothat these prior art solutions involve considerable manufacturingdifficulties. In the last mentioned arrangement also the mechanicaladjustment of the distance of the alkali salt piece is difficult andcritical.

It is an object of this invention to provide a selective ionizationdetector having an inexpensive and very slowly decreasing alkali source,which therefore maintains a substantially uniform detector sensitivityover long periods of time, for instance, months. a

It is another object of this invention to provide a selective nitrogendetector without requiring expensive mechanical means for distanceadjustment and mechanical adjustments at the detector, which is hot andnot very accessible.

Starting from a selective ionization detector of the type mentionedhereinbefore the basic feature of the invention resides in the fact thatthe alkali glass is in a viscous or softened state during detectoroperation.

According to the invention the alkali source therefore consists of glasswhich softens during operation, whereby it is attained that the surfaceof the alkali source does not become impoverished as to alkali, since bymolecular movement alkali is constantly supplied to the surface. In thismanner, an ionization detector according to this invention differsadvantageously from a detector according to the German patentspecification No. 907,223 where the alkali cylinder remains in its rigidstate and therefore an impoverishment of alkali at the surface soontakes place. Since only very small alkaki quantities are consumed, thesupply of a single glassdrop is sufficient for many months. The alkaliglass drop may then be readily replaced. No manufacturing costs formonocrystals, supply vessels, or the like are encountered.

The detector may be of the form that includes a burner nozzle to which acombustible gas and sample gas mixture are supplied, that above theburner nozzle an alkali glass body is mounted and is heatable by anelectric resistance heater to an extent sufficient for softening of thealkali body.

The rate of alkali emission very markedly depends on the temperature ofthe glass. When the heating of the alkali glass body is effectedelectrically and not by the flame, it is not necessary to maintainconstant the flame temperature, and therefore the combustible gas (Hflow, with the otherwise required accuracy. Upon adjustment of the gasflows, no notice need be taken of the effects of flame heating of thealkali glass body, but rather the gas flows of hydrogen and oxygen canbe adapted to the respective temperature requirements, in particular, ofthe element desired to be detected.

Advantageously, the heating of the electric resistance heater isprecisely adjustable. However, provision can be made that independentlyof the precise adjusting means, a fixed heating capacity sufficient forigniting the flame can be switched on by actuation of a push button orthe like. Thus, the resistance heater for the alkali glass body isadditionally used for igniting the flame.

A specificity for particular individual substances can be obtained by asuitable selection of the gas flows and by selection of the appropriatealkali component. By using, for instance, rubidium-containing glass itis possible to detect nitrogen compounds; while, for substances of highphosphorus content, sodium-containing glass is selective.

An advantageous selective ionization detector of the type indicatedabove includes a burner nozzle to which a combustible gas and sample gasmixture are supplied; above the burner nozzle an alkali glass body,heatable up to its softening temperature, is mounted; above the alkaliglass body a collecting electrode is arranged; and the collectingelectrode and nozzle are connected to a positive electric potential withrespect to the alkali glass body. With such a potential distribution theion currents (normal flame ionization detector signals) originating fromthe normal hydrocarbon (CH) components and the ion currents occurring onthe surface of the alkali glass body specifically with the occurrence ofhalogen or phosphorus are separated from each other in a manner knownper se (German published patent application No. l,805,776). Theelectrons from the normal (hydrocarbon) combustion process flow to theburner nozzle, since the mount of the alkali glass body is at apotential negative relative to the nozzle and the electrons cannotovercome this potential. The electrons from the strictly thermionicprocess with the occurrence of, for instance, halogen, occur on thesurface of the alkali glass body and are therefore picked up by thecollecting electrode.

In a further modification of this invention, the nozzle and a housingwhich encloses the nozzle, the alkali glass body, and the collectingelectrode are all provided with insulation with respect to ground; anegative voltage is connected to the housing (with respect to ground)and the alkali glass body is electrically connected to the housing,while the collecting electrode is mounted in the housing, but iselectrically insulated with respect to the same and is connected to anamplifier at ground, and that the nozzle is electrically connectablealternatively to the housing (i.e., negative) or to ground by means of aswitch.

Then, the detector can be operated by means of operation ofa simpleelectric switch alternatively as selective ionization detectoror asnormal flame ionization detector.

A few iiiustrative embodiments of this invention will now be describedmore fully with reference to the accompanying drawings, in which:

FIG. 1 schematically illustrates the design of a selective ionizationdetector incorporating the invention;

FIG. 2 diagrammatically illustrates one possibility of electricalpotential distribution in the ionization detector of FIG. 1 in which thelatter operates as a flame'ionization detector;

FIG. 3 diagrammatically illustrates electrical potential distribution inthe ionization detector of FIG. 1 in which the latter operates as anionization detector selectively responding to phosphorus;

FIG. 4 illustrates a chromatogram recorded with the detector in the modeof operation as shown in FIG. 2;

FIG. 5 illustrates a chromatogram of the same mixture (on a changedscale) in the mode of operation as shown in FIG. 3, the mixture having aphosphoruscontaining component and a non-phosphoruscontaining component;and

FIG. 6 illustrates an embodiment of the ionization detector, permittingalternatively a mode of operation according to FIG. 2 or one accordingto FIG. 3.

The detector according to FIG. 1 is substantially in the form of a priorart flame ionization detector (FID). A housing 10 is divided by apartition 12 into a lower and an upper chamber 14, 16, respectively.Into the lower chamber 14 a burner nozzle 18 protrudes, terminating justbelow an aperture 20 in the partition 12. A sample gas is supplied tothe nozzle 18 via a conduit 22. When applied to gas chromatography, thisconduit 22 is connected with the outlet of a separating column, and thesample gas consists of a mixture of carrier gas plus eluted samplecomponents. A combustible gas, generally hydrogen, is added to thesample gas via a conduit 24, so that a combustible gas, sample gasmixture issues from the nozzle and during operation commonly burns witha flame 26. Combustion air is intro duced into the lower chamber 14 viaan air supply conduit 28.

In the upper chamber 16 above the burner nozzle 18, a bead 30 of analkali-rich glass is mounted, specifically by means of two wires 32 and34 which are passed through the wall of the housing 10 by means ofelectrical insulators 36, 38. These wires 32 and 34 simultaneously serveas current leads for an electric resistance heating element 40 arrangedtherebetween and in heatconducting contact with the bead 30. The heatingquantity of the electric resistance heating element 40 is preciselyadjustable by conventional means not illustrated.

Above the bead 30 is positioned a collecting electrode 42. Thecollecting electrode 42 is secured to an electrically conducting holder44, which is passed downwardly through an aperture 46 of the partition12 and laterally out of the housing 10 by means of an insulator 48. Theburnt gases of the flame 26 are exhausted via a connector 50. When usingthe detector as a leak detector a suction pump is connected here.

The electric resistance heating element 40 is so constituted that thealkali glass bead 30, when the detector is in operation, can bemaintained in a viscous or softened state. In this state, a constantmolecular movement and therefore a concentration compensation takesplace in the alkali glass bead, so that fresh alkali metal atomsconstantly migrate to the surface of the alkali glass bead, andtherefore no impoverishment (of alkali metal) occurs.

Alternatively, it is also possible to suspend the alkali glass bead froma bracket 52 which is mounted on the partition, so that the bead issuspended in the flame 26 and is softened by the latter.

The bracket 52 and/or the wires 32, 34 preferably consist of platinum,on the one hand because platinum has substantially the same coefficientof thermal expansion as glass, and on the other hand due to its chemicalinertness. The partition 12 ensures in well-known man ner that theinsulator 48 and the conduit lead-out are not impaired (i.e., attacked)by combustion residues.

The just described detector is particularly suited for,

the detection of halogenand phosphorus-containing substances. Moreover,in addition to its selective signal it also supplies a normalFID-signal. In order to obtain a signal of high selectivity, the samplemay be burnt before introduction to the alkali source, which can beeffected in a known manner in the first section of a double-level flameionization detector (FID) or by flameless oxidation.

Another possibility of alternatively obtaining a high selectivity isshown by a comparison of FIGS. 2 and 3.

FIG. 2 illustrates a circuit in which the normal FlD'signals resultingfrom the (hydro-carbon) combustion are also obtained. The burner nozzle18 and the alkali source (softened alkali glass bead 30) are connectedto a negative potential with respect to ground by a voltage source 54.The collecting electrode 42 connects to a grounded amplifier 56, thuspractically at ground potential. With this mode of circuit arrangement,both selective signals, for instance, for halogen and phosphorus fromthe thermionic ion currents which originate from the bead 30, and alsonormal FID-signals, for instance, from the hydrocarbon components areobtained. Though, in this mode of operation the relative sensitivitywith respect to halogen and phosphorus is in general increased, however,the detector is not sensitive specifically only to these substances.Such a performance of the detector may be desirable. An increase in thesensitivity is obtained for nitrogencontaining compounds if the flame isoperated under reducing conditions.

In the electrical potential distribution according to FIG. 3, thecollecting electrode 42 again connects to the amplifier 56 with respectto ground. However, now the burner nozzle 18 is grounded, and only thealkali glass bead 30 and its mount are maintained at a negativepotential (with respect to ground) by the voltage source 54.

Normal (hydrocarbon) combustion takes place in the flame below thealkali source, i.e., the bead 30. If the burner nozzle 18 is at apositive potential relative to the bead 30 the electrons from thecombustion process will flow to the nozzle because they cannot overcomethe negative potential of the bead 30 (and of its mount). The electronsfrom the strictly thermionic process, however, are produced on thesurface of the bead 30 above the flame 26 and are therefore picked up bythe collecting electrode 42. This thermionic process thereforesubstantially solely supplies the signal across the amplifier 56.

The correctness of this assumption is substantiated by the actualchromatograms of FIGS. 4 and 5. Both chromatograms were recorded withthe same substance mixture, first (FIG. 4) with the electrical potentialdistribution of the detector as shown in FIG. 2, and secondly (FIG. 5)with the electrical potential distribution shown in FIG. 3. Of interestare the peaks of malathion, a phosphorus-containing substance, and ofcicosan, a 30-carbon hydrocarbon. While during normal FID-operationaccording to FIG. 4 (with the malathion peak at a gain factor of 8) astrong eicosan-peak is recognizable, this latter peak has disappearedcompletely in FIG. 5 made with a grounded burner nozzle 18. From this,the gain in specificity can be seen very instructively, and no decreasein the sensitivity of the malathion peak is recognizable. With thisconstruction a double-level arrangement of the FID is not required.

FIG. 6 illustrates an embodiment which permits the alternative operationaccording to FIG. 2 or according to FIG. 3 in a simple manner.Corresponding parts are referenced by the same reference numerals as inthe FIGS. l to 3.

The burner nozzle 18 is insulated with respect to ground by aninsulating piece 58. Burner nozzle 18, alkali glass body 30 andcollecting electrode 42 are enclosed by a cylindrical housing which ismounted for insulation with respect to ground. The collecting electrode42 is mounted in this housing 60 by means of an insulator 62. By meansof the voltage source 54 the housing 60 is connected to a negativepotential, for instance -l30 volts with respect to ground. By means ofswitch means 63 the burner nozzle 18 can be alternatively connectedelectrically to the housing 60 and therewith to the negative potentialor to ground. The alkali glass body 30 and its conduits are electricallyconnected to the housing 60.

The detector of the invention described can be used in various mannersin order to obtain selectivity for various substances. I

The glass bead 30 can be formed of sodiumcontaining glass and be excitedto alkali emission by electric heating, while the flame 26 burnsreducingly (O supplied at 40 ml/min; H at 15 ml/min). Then, a selectivephosphorus signal is obtained. When using rubidium-containing glass aselective nitrogen (N signal can be obtained by corresponding adjustmentof the gas flows (0 at 0 ml/min, H at 5 ml/min) when the glass bead 30is heated electrically. The hydrogen does not operate a flame.

Generally, the detector can be operated in the following ways:

I. As a normal flame ionization detector (FID), air and hydrogen aresupplied, the flame 26 burns, and the electric resistance heatingelement 40 is inoperative. The operating conditions are for instance: N(carrier gas) at 25 ml/min; 0 at 300 ml/min; H at 30 ml/min.

2. As FID with selective sensitivity for halogen and phosphoruscompounds when a reducing (burning) an electrical potential distributionaccording to FIG. 3 (with grounded nozzle 18) is utilized.

4. As FlD with conventional sensitivity and additional sensitivity forhalogen and phosphorus, if the glass bead 30 is arranged on the bracket52 in the flame 26 and is heated by the flame. The operating conditionsare for instance: N at 25 ml/min; at 380 ml/min; H at 40 ml/min.

5. As selective detector for nitrogen compounds, if no oxygen issupplied to the detector, and the flame does not burn, but through theelectric resistance heating 40 a heavy heating current flows. Theoperating conditions are for instance: N at 25 ml/min; H at 5 ml/min;heating current (I) 2.5 amps.

Advantageously, the resistance heating 40 is also used for igniting theflame 26 when operation of the detector is started. For this purpose, apush button switch (not shown) may be provided by which, independentlyof the precisely effected adjustment of heating quantity (to element40), a fixed amount of heating can be applied to the resistance heatingelement 40 which is fully sufficient to ignite the flame 26.

We claim:

1. A selective ionization detector of the type for detecting halogen,phosphorus and nitrogen compounds, comprising a diode through which asample gas under analysis is fed by means of a transfer gas, and anelectrode including an alkali source in the form of a heatedalkali-containing glass, so that the electrode exhibits an increased ionemission upon occurrence of such specific substances, the improvement inwhich:

said alkali source comprises alkali glass maintained in a heatedsoftened state during operation of the detector.

2. A selective ionization detector as claimed in claim 1, in which:

said alkali glass is a sodium-enriched glass.

3. A selective ionization detector as claimed in claim 1, in which:

said alkali glass is a rubidium-enriched glass.

4. A selective ionization detector as claimed in claim 1, furthercomprising:

a burner nozzle (18) to which a combustible gas and sample gas mixtureis supplied:

an alkali glass body (30) heatable up to softening mounted above saidburner nozzle (18); a collecting electrode (42) arranged above saidalkali glass body (30); said collecting electrode (42) and burner nozzle(18) being connected to an electrical potential positive with respect tothe alkali glass body (30). 5. A selective ionization detector asclaimed in claim 4, in which:

a housing (60) encloses said burner nozzle (18); said alkali glass body(30) and said collecting electrode (42) are mounted by means ofinsulation (58) with respect to ground; said housing (60) is connectedto a negative voltage source with respect to ground; said alkali glassbody (30) is electrically connected to said housing (60); saidcollecting electrode (42) is mounted in the housing (60) but iselectrically insulated therefrom and is connected to an amplifier (56)which is grounded; and said burner nozzle (18) is electricallyconnectable alternatively to the housing (60) or to ground by means of aswitch means. 6. A selective ionization detector as claimed in claim 1,further comprising:

a burner nozzle (18) to which a combustible gas and sample gas mixtureis supplied; said alkali source is in the form of an alkali glass body(30), mounted above said burner nozzle (18) and heatable by an electricresistance heating means (40) to an extent sufficient for softening ofsaid alkali glass body (30). 7. A selective ionization detector asclaimed in claim 6, in which:

the heating energy supplied to the electric resistance heating means(40) is precisely adjustable. 8. A selective ionization detector asclaimed in claim 7, in which:

independently of said precisely adjustable energy, a separate heatingquantity sufficient for igniting the flame can be supplied to saidheating means by actuation of a manually operated means.

1. A SELECTTIVE IONIZATION DETECTOR OF THE TYPE FOR DETECTING HAOLGEN,PHOSPHORUS AND NITROGEN COMPOUNDS, COMPRISING A DIODE THROUGH WHICH ASAMPLE GAS UNDER ANALYSIS IS FED BY MEANS OF A TRANSFER GAS, AND ANELECTRODE INCLUDING AN ALKALI SOURCE IN THE FORM OF A HEATEDALKALI-CONTAINING GLASS, SO THAT THE ELECTRODE EXHIBITS AN INCREASED IONEMISSION UPON OCCURRENCE OF SUCH SPECIFIC SUBSTANCES, THE IMPROVEMENT INWHICH: SAID ALKALI SOURCE COMPRISES ALKALI GLASS MAINTAINED IN A HEATEDSOFTENED STATE DURING OPERATION OF THE DETECTOR.
 2. A selectiveionization detector as claimed in claim 1, in which: said alkali glassis a sodium-enriched glasS.
 3. A selective ionization detector asclaimed in claim 1, in which: said alkali glass is a rubidium-enrichedglass.
 4. A selective ionization detector as claimed in claim 1, furthercomprising: a burner nozzle (18) to which a combustible gas and samplegas mixture is supplied: an alkali glass body (30) heatable up tosoftening mounted above said burner nozzle (18); a collecting electrode(42) arranged above said alkali glass body (30); said collectingelectrode (42) and burner nozzle (18) being connected to an electricalpotential positive with respect to the alkali glass body (30).
 5. Aselective ionization detector as claimed in claim 4, in which: a housing(60) encloses said burner nozzle (18); said alkali glass body (30) andsaid collecting electrode (42) are mounted by means of insulation (58)with respect to ground; said housing (60) is connected to a negativevoltage source with respect to ground; said alkali glass body (30) iselectrically connected to said housing (60); said collecting electrode(42) is mounted in the housing (60) but is electrically insulatedtherefrom and is connected to an amplifier (56) which is grounded; andsaid burner nozzle (18) is electrically connectable alternatively to thehousing (60) or to ground by means of a switch means.
 6. A selectiveionization detector as claimed in claim 1, further comprising: a burnernozzle (18) to which a combustible gas and sample gas mixture issupplied; said alkali source is in the form of an alkali glass body(30), mounted above said burner nozzle (18) and heatable by an electricresistance heating means (40) to an extent sufficient for softening ofsaid alkali glass body (30).
 7. A selective ionization detector asclaimed in claim 6, in which: the heating energy supplied to theelectric resistance heating means (40) is precisely adjustable.
 8. Aselective ionization detector as claimed in claim 7, in which:independently of said precisely adjustable energy, a separate heatingquantity sufficient for igniting the flame can be supplied to saidheating means by actuation of a manually operated means.